Various organs of a human body are lossy dielectrics, so that electromagnetic field in human body will generate currents that absorb and dissipate electromagnetic energy. This physical process is usually represented as the specific absorption rate (SAR) in the biological dosimetry.
The SAR refers to the electromagnetic power absorbed or dissipated by per unit mass of human tissues, and has a unit of watts per kilogram (W/Kg).
Mobile phones radiate a great amount of electromagnetic waves to the outside during use, and the mobile phone are generally putted beside his/her ear during a phone call, therefore the long-term use of the mobile phones will cause brain dysfunction and influence human health. Thus, it is necessary for the mobile phones to pass an SAR test before shipment.
Currently the SAR test tests the SAR mainly by disposing the mobile phone at 2 to 3 centimeters away from a human face.
Various data of the current antenna using a GSM850 frequency band will be described with reference to table 1:
TABLE 1RadiationMaximumpower of theoutput power ofDAC value ofantennaRFPARFPASARData28.2 dBm    33 dBm5801.2 W/KgStandard  28 dBm33 ± 2 dBm<10231.0 W/Kg
As shown in table 1, the radiation power of the antenna using the GSM850 frequency band is 28.2 dBm, and the standard value is specified to be higher than 28 dBm. The standard value of the SAR is 1.0 W/Kg, but the SAR of the GSM850 has reached 1.2 W/Kg.
The radiation power of the antenna of a mobile phone is directly related to the SAR. The higher the radiation power of the antenna is, the greater the SAR will be. Therefore, one of the approaches to reduce the SAR is to reduce the radiation power of the antenna. Therefore, the SAR can be reduced by directly reducing the output power of the radio frequency power amplifier (RFPA) so as to reduce the radiation power of the antenna.
However, a resulting problem is that: if the maximum output power of the RFPA is reduced, the radiation power of the antenna will be reduced simultaneously. If the maximum output power of the RFPA is reduced to 32 dBm, then the radiation power of the antenna will be reduced by about 1 dBm simultaneously. Consequently, the radiation power of the antenna cannot meet the standard when it is tested in a free space.
Thus, when the radiation power of the antenna is adjusted to meet the standard, i.e., higher than 28 dBm, during related tests on the mobile phone, the SAR will be found to reach 1.2 W/Kg, which is out of the limit, in the SAR test. In this case, the antenna radiation power test and the SAR test cannot meet the standard simultaneously by directly reducing the output power of the RFPA.